The ability of the cyanide ion (CN--) to complex metal ions has been used to prepare a wide variety of so-called coordination complexes or coordination compounds hereinafter described as metallocyanide complexes. The most well known of these complexes are those of the ferrous ion Fe(II) and the ferric ion Fe(III). The cyanide complexes of these ions can best be described in terms of the formula for the hydrogen ferrocyanide EQU H.sub.4 [Fe(CN).sub.6 ]. (I)
all or part of the hydrogen of this compound may be replaced by a metal ion or by a cation such as ammonium (NH.sub.4.sup.+), or may remain in the compound. The complex may thus have the general formula EQU H.sub.(4-n) M'.sub.m'/v' M".sub.m"/v". . . [Fe(CN).sub.6 ] (II)
in which n is greater than or equal to 0 to 4 (0 .ltoreq. n .ltoreq. 4), m', m", . . . are the coefficients of the cations M', M" . . . representing the metals or ammonium ion, and v', v" . . . are the valences of the respective cations. It will be apparent that m' + m" + . . . = n.
In an analogous manner, the ferricyanide complexes derived from hydrogen ferricyanide EQU H.sub.3 [Fe(CN).sub.6 ] (III)
can be described as a compound of the following general formula EQU H.sub.(3-n) M'.sub.m'/v' M".sub.m"/v". . . [Fe(CN).sub.6 ] (IV)
in which m', m", v', v", M', M" etc have the same definitions as given above except that 0 .ltoreq. n .ltoreq. 3.
These compounds are blue pigments widely used in many fields and identified as PRUSSIAN BLUE and TURNBULL BLUE. Various other cyanide compounds have been described in the literature for iron and other elements of the Periodic System. For example, apart from the single cyanide represented as Ln(CN).sub.3, where Ln represents the members of the lanthanide series of elements with atomic numbers of 57 to 71, inclusive, ferrocyanides of the lanthanide metals and alkali metals of the general formula M'Ln[FeCN.sub.6 ] are known, Ln being the general symbol for a lanthanide metal and M' representing an alkali metal such as sodium or potassium. The lanthanide ferrocyanides Ln[Fe(CN).sub.6 ] are also known in the art as are the lanthanide nickelocyanide Ln.sub.2 [Ni(CN).sub.4 ] and the lanthanide cobalticyanides Ln[CoCN.sub.6 ]. For the most part these compounds are useful as pigments, are slightly soluble in water and may be obtained by precipitation from water by virtue of their low solubility products.